Donald Trump is launching a serious challenge to Gov. Cuomo — but it’s not in the race for governor that’s coming up next year.

The megabuilder says he’s ready to take over Cuomo’s massive $4 billion Tappan Zee Bridge construction project, getting the job done “for peanuts’’ by rebuilding, rather than replacing, the existing structure across the Hudson River.

“It’s just like the Wollman Rink in Central Park [in 1986]: They had it under construction for eight years. I told Ed Koch I want to take it over, get it done quickly. I got it done in three months for $1.8 million,’’ Trump told The Post.

Donald TrumpDonna Ward/Getty Images

“I could fix the Tappan Zee Bridge for relative peanuts. I could get it done for so little and with so little disruption that it would make your head spin,’’ Trump continued.

Cuomo, who last week announced federal approval for a $1.6 billion loan for the already-underway bridge project, has insisted a new Tappan Zee was needed because the existing span, opened in 1955, was well beyond long-term repair.

“They had some engineer who said it can’t be fixed. You know why? Because they want to build a new bridge. That’s what they want to do,’’ Trump contended.

“They said its useful life is 50 years? So you extend that for another 50 years. That’s what construction is all about. That’s what I do . . . $4 billion? It could be fixed for a tiny fraction of that. Nobody builds better than I build.

“We have the highest taxes in the nation, and Cuomo is building a bridge where the tolls may be $25? It doesn’t make any sense.’’

Neither Cuomo nor the state Thruway Authority, which owns the bridge, has specified how high the tolls will be once a new Tappan Zee is completed.

Trump says he’s weighing running as the Republican candidate against Cuomo next year, although in late October he said he had no interest in doing so.

GOP insiders say they expect Trump to visit New Hampshire next month to again focus on a possible run for the presidency rather than for governor.

Cuomo has privately told groups of “progressive’’ Assembly Democrats that despite his election-year plans to reduce some state taxes, he’s sure he can find funds to help soon-to-be-Mayor Bill de Blasio deliver on his pledge of universal pre-K throughout the city.

Cuomo, in recent meetings he initiated with black and Hispanic caucus members and others, appeared to fear that city Democrats sympathetic to de Blasio’s attacks on the wealthy and angry over his Moreland Act Commission probe of state lawmakers, might turn on him during the next legislative session, a source familiar with the meetings said.

“Many members like de Blasio’s proposals and blame Cuomo for demonizing the Legislature, for instance, putting out stories that members got a $500 contribution from someone or some group — when the governor has gotten $50,000 from the same person or group.

“The meetings show the governor realizes he’s going to have a problem next year when he’s running for reelection and that members aren’t going to be as responsive to his needs as they have been in the past,’’ said the source.

Sen. Jeff Klein of The Bronx, whose small block of “independent” Democrats have allow Republicans to keep control of the Senate, is “scared and desperate’’ that he’ll be defeated in a primary challenge next year, a Senate insider has told The Post.

Klein, who once bill himself as a “moderate’’ when compared to the left-of-center activists who dominate Senate Democrats, has recently embraced de Blasio’s call for higher taxes on the wealthy while appearing to reject the governor’s call for tax breaks to help suburban and upstate residents.

“Klein sees the tide running with the ‘progressives,’ and he’s throwing whatever principles he may have had overboard in order to go with it,’’ the insider said.